1. Field of the Invention
The present invention relates to a circuit breaking system, and more particularly relates to a circuit breaking system having a means for cutting off power to a control device and detecting a failure, when there is a short between a drive power supply and a constant power supply, in a circuit that cuts off power to drive the control device according to safety conditions such as emergency stop and servo off.
2. Description of Related Art
Drive power supplies for control devices use a circuit that cuts off power by double contacts to make sure the power is properly cut off. In this case, when there is a short between another power supply (power supply that is not cut off by the double contacts) and the drive power supply in a position beyond the double contacts, the drive power supply is supplied with power even after disconnecting the double contacts. Although the short between the power supplies can be detected by monitoring a voltage, the power supply by the short can not be disconnected, thus posing a danger.
FIG. 1 is a block diagram of a conventional device control system 1000. A power output terminal 41 of a power supply 4 is connected to a constant power supply terminal 32 of an output device 1 through a wire L1, and connected to a power input terminal 3 of the output device 1 through a wire L2. Thus, the power supply 4 supplies the output device 1 with electric power (for example, 24 [V]). A protection circuit (breaker) QF1 is disposed between the power output terminal 41 and the constant power supply terminal 32. A protection circuit (breaker) QF2 and double switching circuits (KM1, KM2) are disposed between the power output terminal 41 and the power input terminal 3. Another terminal 42, different from the power output terminal 41, of the power supply 4 is grounded.
The switching of the switching circuit KM1 is controlled by a first operating coil 71, which is operated by a signal from an output terminal DO of a first CPU 61. The switching of the switching circuit KM2 is controlled by a second operating coil 72, which is operated by a signal from an output terminal DO of a second CPU 62. The first and second operating coils 71 and 72 also control the switching of contacts of an electromagnetic contactor 200 disposed between a servo power supply 100 and a servo amplifier 300.
In the device control system 1000 shown in FIG. 1, when there is a short between the constant power supply terminal 32, which is not cut off by the double contacts, and the power input terminal 3 in a position beyond the double switching circuits KM1 and KM2, electric power is supplied from the constant power supply terminal 32 to the power input terminal 3, i.e., a drive power supply, even after disconnecting the double switching circuits KM1 and KM2.
The short occurring between the constant power supply terminal 32 and the power input terminal 3 will be described in detail. FIG. 2 shows an example of normal wiring between the output device 1 and a first control device 21 and between the output device 1 and a second control device 22. For example, the constant power supply terminal 32 and the power input terminal 3 are each branched off to a plurality of terminals in a branch terminal block, and connected to the first control device 21 and the second control device 22. Inside the first control device 21, a conductor 210 connects two of the terminals connected to the power input terminal 3.
FIG. 3 shows an example of miswiring between the output device 1 and the first control device 21 and between the output device 1 and the second control device 22. Miswiring occurs when, for example, the conductor 210 of the first control device 21 is wrongly connected to the constant power supply terminal 32 at one end, as shown by a dashed line, while being connected to the power input terminal 3 at the other end. The miswiring may cause a short circuit between the constant power supply terminal 32 and the power input terminal 3.
Such a short between the power supplies can be detected as a defect by connecting an input terminal DI of the first CPU 61 to the wire L2 and monitoring the voltage of the power input terminal 3, as shown in FIG. 1. However, the short circuit between the constant power supply terminal 32 and the power input terminal 3 cannot be solved. That is, even if both of the switching circuits KM1 and KM2 are in an OFF state, 24 [V] is outputted to the power input terminal (OUTP) 3, thus failing to cut off power to the power input terminal 3.
Also, circuit breakers to break an electric circuit upon the occurrence of a defect are known (for example, Japanese Unexamined Patent Publication (Kokai) No. 2011-016391, hereinafter called “patent document 1”). A circuit breaker disclosed in the patent document 1 has problems that a break circuit has to be used together with a semiconductor circuit to control the break circuit, and the addition of a failure detection circuit for the break circuit is required.